Process for carbonizing cellulosic fibrous substrates

ABSTRACT

A process for improving the thermal carbonization of cellulosic substrates comprising treating said substrates prior to carbonization with a catalytic amount of at least one compound selected from esters of phosphoric acid, esters of phosphorothioic acid, phosphoramidic acids, alkylphosphoramidic acids, alkylphosphonic acids, alkylphosphonothionic acids, and phosphonitrile amide derivatives and heating said treated substrates until a carbonized product having a carbon content of at least 50 percent weight carbon is produced.

United States Patent Donald R. Moore Rutherford;

Stanley E. Ross, Passaic; Giuliana C. Tesoro, Dobbs Ferry, all of NJ.

Inventors App]. No. 19,488 Filed Mar. 13, 1970 Division of Ser. No. 721,145, Apr. 15, 1968, Pat. No. 3,527,564. Patented Nov. 2,1971 Assignee J .1. Stevens & Co., Inc.

New York, N.Y.

PROCESS FOR CARBONIZING CELLULOSIC FIBROUS SUBSTRATES 3 Claims, No Drawings U.S. Cl 23/209.5, 8/116, 8/1 16.2

Int. Cl C01b 31/07 Field of Search 23/2095,

[56] References Cited UNITED STATES PATENTS 3,235,323 2/1966 Peters 8/116.2 3,294,489 12/1966 Millington et al 23/2094 OTHER REFERENCES Schmidt et a1. Chem. Eng. Progress" vol. 58, No. 10, Oct. 1962, pages 42- 43 Primary Examiner- Edward J. Meros AttorneysMichael T. Frimer and Charles Stein carbon is produced.

PROCESS FOR CARBONIZING CELLULOSIC FIBROUS SUBSTRAT ES This application is a division of copending [1.8. application Ser. No. 721,145, now US. Pat. No. 3,527,564 filed Apr. 15, 1968.

This invention concerns reagents useful as catalysts for the pyrolytic transformation of carbon-containing substrates to carbon products.

More particularly, this invention relates to novelcatalysts employed during the carbonization and graphitization of cellulosic substrates to effect product and process improvement.

Carbonization as used/herein refers tothermally actuated processes which transform a carbon-containing substrate to a carbon product. These carbon products include both carbon and graphite materials whose carbon (or graphite) content ranges from about 50 percent by weight to almost 100 percent by weight.

The term cellulosic as used herein refers to substrates comprising natural cellulose or its modified derivatives. These include cotton, linen, hemp, jute, flax, wood,.cuprammoni um rayon, viscose and the like. These cellulosics can: be employed unblended or in the form of blends with noncellulosic materials. The latter include, but are not limited to the acrylics, polyesters, and polyamides. The substrates can be used in. the form of their yarns, fibers, or threads as well as in theform of materials such as woven cloths, knitted fabrics, webs, laminates, or any other fabricated form utilizing textile fabricating processes.

The carbonization of carbon-containing substrates to produce carbon or graphite filaments dates back more than 60 years. More recently, stimulated by the need in the aerospace program for strong, lightweight and inert materials, there. has been a resurgence of carbonization and graphitization research. Particularly of interest are programs in which.carbon-containing textile materials such as cellulosic yarn .and cloth have been pyrolyzed .undercarefully controlled conditions to produce carbon products retaining a substantial-portion of the desirable characteristics of the original textile substrate. For example, the products are lightweight,have good flexibility, and are nontoxic. Further, as compared to -.the original cellulosic substrate,.they have improved chemicallinertness and dimensional stabilitysFinally, in contrast tolthe cellulosics, they possess outstanding resistance to flammability and good high-temperature ablation.

Most of the published procedures for carbonizing cellulosics disclose lengthy stepwiseheating processes toproduee the final product. Initially, the substrate is slowly heated to temperatures ranging from about 200? C. to 30'C.'followed by a final and brief heating step at high temperatures ranging from about 550 C. to 2,000 C. or even higher. Not only are equipment and labor costs highbut theprolonged heating. at elevated temperatures reducesyields and requires-the use of inert atmospheres to prevent combustion. of the substrate. As a result of these factors, the cost of carbonized articles is exceedingly high and the widespread use of the carbonproducts has been kept to a minimum.

It has been known for some time that pretreatment of the cellulosic substrate with appropriate catalytic reagents can reduce the'time required for completion of the carbonization cycle. Recently, the use of certain mineralacids and acidic salts such as phosphoric acid and diammonium hydrogen phosphate as carbonization catalysts have been described'in patents (U.S. Pat. Nos.;3,235,323 and 3,305,315). The patentees have indicated that the use of these catalysts during carbonization produces improved carbon products compared to products obtained without'catalysts. However, evenwhen a superior catalyst such as diammonium hydrogen phosphate [(NH,,) HPO,)] is used, difficulties are encountered. Forexample, even when the catalyst is used, a comparatively lengthy heating cycle is required-to obtain a carbonizedproduct having acceptable properties. Further, diammonium hydrogen phosphate should be applied at near boiling temperatures. A major advance in the art would be the development of new catalytic reagents which could be applied at moderate temperatures (20 C.50C.- and which would substantially .reduce the heating time required for carbonization compared to diammonium hydrogen phosphate. In addition, a truly superior catalyst would lend itself to simple, routine application techniques and would produce a.,carbon producthaving improved physical'properties in,good yield. In addition, the

catalyst should be relatively simple to. prepare and'purify, and

would function at effectively low concentrations.

It is, therefore, an object of this invention tojdisclose a novel andsuperionclass of catalysts for the carbonization of cellulosic substrates.

tion of theinventive catalysts to preparecarbon, products having improved properties.

Otherobjects of this invention will become apparent to 'thoseskilled in the-artafter a readingof this application.

The above objects are realized by the treatment of a cellulosic substrate; prior to complete carbonization with a class of carbonization catalysts to be described more fully below.

In practice, a cellulosic substrate is treated with one or more catalysts-selected from the groups consisting of:

wherein G. isselected from the group consisting of -OR or R is an alkylra'dioal: having from one to four carbon a-toms,-R' and Rwhichcan-be the-same or differentat any given time areselected 'fromihydrogen and R; L is selected-from the group consisting of alkylradicals containing from one to four carbon atoms,-.ha-logenated alkyl radicals containing from one 'to'four carbon atoms, andG; and Z is selected from .the, group consisting of oxygen and sulfur; in an amount sufficient to in- .corporate at least-a catalytic quantity" of catalyst into the substrateto be carbonized.

*Whileawatalytic'quantity of catalyst is a variable depending upon the catalyst and reaction conditions employed, in general this quantityyaries between and=30% by\weight of catalyst(s).' based on the weight of the substrate.

While the-abovetwo classes of compounds providesatisfactory ,catalysts-for-the carbonization of carbon-containing substrates,-.in any large-group for various reasons some members of the group are preferred to others. ln' the instant case the preferredcatalyst are selectedfrom thegroup consisting of phosphoric triamide, phosphorothioic triamide, phosphonitrile amide cyclic trimer, diethyl methylphosphoramidate, and diethyl ethylphosphorami'date. These compounds are-the preferred catalysts for carbonizing carbon-containing substrates because they reduce the carbonizationztimerrequired in the first heating stage andrproduce carbon products having outstanding properties in most instances in high yield.

A secondary favored group of compounds which, while excellent catalysts, are not as outstanding as the preferred catalysts, are those compounds selected from the group consisting of methylphosphonic diamide and (chloromethyl)phosphonothionic diamide. These catalysts generally give good yields of carbonized products and the products have superior properties. The trialkyl phosphates as exemplified by triethyl and trimethyl phosphate, N,N,N"- trimethylphosphoric triamide, and diethyl isopropylphosphoramidate, form a tertiary group of very good catalysts which give lesser yields of a product having somewhat less desirable properties.

The residium of the compounds included within the two broad classes of catalysts are least preferred for a variety of reasons including poor water solubility, poorer properties, and the requirement for longer residence times.

After treatment with catalyst, the treated substrate is then heated to effect removal of the volatiles to obtain a permanently dehydrated material having a carbon content of at least 50 percent by weight carbon. This black appearing, devolatilized and heat-treated material can be utilized as an electrical insulator or can be further heated to produce a carbonized or graphitized material having a carbon content of at least 85 percent and up to 100 percent by weight carbon. The latter products are useful as electrically conductive materials and when utilized in resin-bonded laminates are exceedingly valuable in fabricating ablative composites.

In the preferred practice, a fibrous cellulosic textile substrate such as cotton or rayon cloth or yarn is treated with an aqueous solution (1-10 percent active solids) of at least one catalyst selected from the above-described, preferred group until a pickup of 150-300 percent based on the dry weight is obtained. The treated cellulosic substrate is dried to a moisture content of approximately 8-12 percent and heated at 200-300 C. until a black fibrous substrate having a carbon content between 50-65 percent by weight carbon is produced. Ordinarily, depending upon the heating temperature, and the efficacy of the catalyst, the initial carbonization takes between 3 and 12 minutes. Substrates treated with diammonium hydrogen phosphate require between 8-12 minutes when carbonized at this temperature range.

As noted earlier, the product containing 50-60 percent by weight of carbon can be further carbonized to produce a product containing 85 percent by weight or higher carbon content. This is most easily accomplished by heating the carbonized substrate briefly above about l,0OO C. in a nonoxidizing atmosphere. The upper temperature range is primarily limited by whether a carbon or graphite product is desired. The precise residence time required for the final carboniza tion is a variable depending primarily on the second carbonization temperature and to some extent on the catalyst employed during carbonization. When phosphoric triamide (one of the preferred catalysts of this invention) is employed as catalyst, good results have been obtained by heating the black uniformly lustrous fibrous substrate for from 20 to 120 seconds at the l,300 C.-l ,500 C. range. After the final carbonization (or graphitization) is complete the fibrous carbon product is cooled, collected and secured or otherwise finished depending upon the intended end use.

As described supra, the preferred carbon-containing substrates are cellulosics, particularly those broadly described as regenerated cellulose or rayon. The fibrous rayon can be in the form ofa cloth (woven, knitted or felted), nonwoven random layed fibrous or needlepunched batts, felts, fabrics or tissues formed of staple, fibers, yarns, rovings, continuous filament tows and the like.

The cellulosic substrate can be in the form of yarns, fibers or filaments (or the textile fabrics made from these) whose diameter can vary widely. An especially suitable range lies between about to about 30 microns in diameter.

As the preferred catalysts of this invention are water soluble, it is most convenient to utilize the one or more catalyst employed in the form of their aqueous solutions. Water solubility is an important attribute of a preferred catalyst since it permits the use of simple application techniques and existing apparatus. However, if desired, emulsions or suspensions of the soluble or less soluble catalyst(s) in nonaqueous solvents or mixtures of nonaqueous solvents with water can be employed. Textile adjuvants such as surfactants, emulsifiers, stabilizers, or the like can be present in the treating solution if desired.

The mode of applying the treating solution is not critical to the success of this invention. Any application technique utilized in the textile art can be employed. These include, but are not limited to, padding, brushing, spraying, coating, and the like. As the catalysts can be applied at moderate temperatures, a convenient mode of applying them to yarn is to run the yarn continuously through an aqueous bath containing catalyst and removing excess solution to achieve the desired wet pickup. When cloth is used as the substrate, comparable application methods may be employed.

After the application is complete the treated substrate is dried. The drying step can be air dried or heat assisted. It can be conducted as a separate step or can be incorporated as part of the heating cycle.

Similarly the carbonization or graphitization can be conducted in two or more stages or as a continuous stage. No special heating source is needed. Conventional muffle or tube furnaces can be used modified or as marketed.

In the initial carbonization phase (up to about 50-60 percent by weight carbon content) the use of the inventive catalysts precludes the need for heating in a nonoxidizing atmosphere to prevent combustion. However, the subsequent high temperature heating step where the carbon content runs from percent by weight and higher requires a nonoxidizing 7 atmosphere (less than 10 percent by weight oxygen). This can be accomplished by heating the substrate in a nitrogen, carbon dioxide, or noble gas atmosphere (or mixtures thereof) or by diluting air with diluents such as steam, ammonia, or the above enumerated inert gases until the desired environment is obtained.

The final carbonized or graphitized carbon product can be washed or scoured to remove undesirable contaminants or salts if desired. In the former case the product can be batchwise or continuously immersed in water while in the latter instance soaps or detergents can be added to the bath if convenient.

Having described the inventive process generally, it only remains to disclose specific embodiments of the use of the catalysts of this invention in carbonization and graphitization procedures. A description of the evaluation techniques precedes the specific embodiments.

I. Criteria Used to Evaluate the Carbonization Catalysts of this Invention In order to evaluate the carbonization catalysts objectively and to compare them in efficacy to the Prior Art Standard (diammonium hydrogen phosphate), the following criteria were used:

A. Length of time required to carbonize satisfactorily to a carbon product when the first stage of carbonization is carried out at a relatively low temperature, i.e., about 260 C.

B. Tensile strength of the substantially carbonized (85 percent carbon or over) yarn.

C. Modulus of elasticity of the substantially carbonized yarn.

D. Percent by weight carbon content of the substantially carbonized product.

E. Electrical resistivity of the substantially carbonized yarn. In all instances, diammonium hydrogen phosphate was used as the control.

A. Specific Two-Stage Carbonization Process Used l. A viscose rayon continuous filament yarn (1,650 denier/720 filaments/22) manufactured by the IRC Division of Midland Ross Corporation was used as substrate. The yarns were usually two-plied and treated with the catalyst being evaluated by running the yarn through a 3-10 percent by weight aqueous solution of the catalyst so that wet pickup of 200 percent (based on dry weight) is obtained.

. The treated yarn is dried in the presence of air at about 120 C. for minutes to produce a treated substrate containing about 8-10 percent by weight moisture.

. The above yarn, which is at its normal moisture regain,

is heated in the presence ofair at about 260 C.*

Where other temperatures were used, they are indicated. for a sufficient period of time to produce a partially carbonized substrate whose carbon content ranges between 50-65 percent by weight carbon.

4. The partially carbonized stibstrate is "63556? bonized in a nitrogen atmosphere at 1,372" C. for seconds to produce a substantially carbonized substrate having a carbon content of about 90 percent by weight or higher.

5. The carbonized yarn is collected and wound on an appropriate storage cone. B. Tensile Strength l. A 60-inch length sample of the aforedescribed viscose yarn which has been carbonized is taken for sampling. Samples of both the 50-60 percent by weight and 85-100 percent by weight carbon content are used. Both samples are conditioned for 16 hours at a relative humidity of 65:2 percent (at 21 C.il C.). The sample is tested on an lnstron Tensile Tester (manufactured by lnstron Engineering Corp.) as follows: The sample is positioned so that the span between the jaws of the'instrument is 5 inches. The rate of extension is 10 percent per minute and the chart speed adjusted so that every inch on the chart equals one-half percent extension. The ultimate breaking strength in gramsis calculated from the chart and used to calculate the tensile strength figure.

. Ultimate Elongation (strain )Using the chart, the ultimate elongation value is determined.

C. Modulus of Elasticity Using the slope of the curve obtained in the tensile strength determination, the modulus, expressed in grams per denier, is determined.

D. Carbon Content A 3-4 mg. sample of carbon yarn, previously dried and pulverized, is weighted out in an aluminum combustionboat and. introduced into a combustion tube of a Coleman Model 33* Carbon-Hydrogen Analyzer. The combustion tube is maintained at 900 C. during ignition. The analyzer is previously calibrated against a sample of chemically pure acetanilide. All samples are run in duplicate.

1 Manufactured b Coleman 16566561115, 13. Zililadison S tr ee t. Maywood,

E. E'i'cifi'cil Resistivity A resistance meter capable of measuring resistance below approximately 50,000 ohms is used. A 2.5 centimeter span of carbonized yarn having a content between 85-100 percent by weight carbon is used as sample. The sample is removed and tested immediately after carbonization. Three readings are taken over the span of the yarn and these readings are averaged to give the resistance figure expressed in ohms per centimeter. The resistance value and the cross sectional area are used to calculate the specific resistivity. A resistivity value of 20 ohm-cm. or less is considered satisfactory for carbonized yarns containing between 85-100 percent by weight carbon, all of the preferred catalysts produced yarns having resistivity values within this value. 11. Source of Catalysts All of the catalytic reagents of this invention but one* '(chloromethyl)phosphonothionic diamide are known compounds. A description ofthe source of the preferred reagents is indicated below. Where the reagents were synthesized a reference to the literature or a description of the preparation is provided.

Catalyst Phosphorothioic trianlide:

Source Synthesized according to the method described in Inorganic Syntheses, vol. VI, pp. 111-112 (1969).

Synthesized according to the method of Shaw and Ogawa described in Journal Phosphonitrile amide cyclic trimer:

N=P(NH2)2 of'Polymcr Science, part A, vol. 3,

pp. 3343-3351 (1965), where it was (H2N)2P N called "hezaaminocyclotriphosphazatriene." The systematic nan-c is NP (NH?) 2 2,2,4,4,6,(rhe za amino -2 ,2 ,4, 4 ,6,6-heznhydro-1,3,5,2,4,6-triazatriphosphorine.

Synthesized according to the method of Ratz described in .7. Am. Chem. 800.. vol. 77, p. 4,170 (1955).

Methylphcsphonic diamidc:

O P (NH 2) 2 (Chloromethyl)phosphono- See preparation below: 1 thiouic diamide:

ClCH:

1 S P (N112) 2 A li-liter reaction flask is charged with 3 liters of chloroform (dried over C8012), and ammonia gas is bubbled into the solvent for 3 hours at -15 0. During the introduction of the ammonia, 183.4 g. (1.01nolc) of (chloromethyl)phosphonothioic dichloride is added dropwise to the flask at -15 C. over a 2-hour period. After the addition of dichloride is completed the mixture is stirred at 15 C. and ammonia is bubbled through for an additional one hour. The reaction mixture is then allowed to come up to room temperature overnight. The mixture is filtered and the white solid is collected and dried. The filtrate is evaporated and the residue combined with the collected solids. The combined solids are refluxed with a 75:25 chloroform-methanol mixture to extract the product. The extract on cooling gives white crystals which are collected and dried and he liltrate is used to extract more product to give a. total 01 116 a. of the product, (chloromethyl)phosphonothionic diamide in 66.5% yield. M .P. 107-109 C. The analyses were as follows:

Elements Calculated Found C 8. 35 8. 18 H 4. 15 3. 74 N 19. 3 19. 2 P 21. 4 22. 9

The following compounds used as catalysts were purchased from a commercial source:

Trimethyl phosphate Triethyl' phosphate Diethyl methylphosphoramidate Diethyl isopropylphosphoramidate 111. Evaluation Results v 7 Using the procedure described in Section 1A, the above compositions-and mixtures thereof were evaluated as-catalysts',

. after carbonizingfor the indicated time at the first heating stage of about 260 C.*

Where temperatures'other than 260 I C. were used they are'indicated in the tabIesDiammonium hydrogen phosphate in all instances was usedas control.

The results using'yarn substrates were as follows:

Breaking Strength-grams 1,304

A. Phosphohitrileaifidgcyeli'e trifii rj' 7 W'Tinil slmielhee-lde Elongation% 1.76 NIP (NHDZ Modulus of Elasticityg./d. 150

| Carbon Content% 93.0 2 )2 N ll 5 NP (NEW E. Phosphorothioic triamide, SP(NH (applied as 3.5 (applied as 3.5% solution). Percent 501mm) After carbonization for the specified times and temperaf s esl ence times at ower tures the resultant yarn had the indicated propertl s. mmpemure 515 min.

Carbonization temperature Pl'OdUCl PTO erties 260 c.-1a72 c. 266 0.437? c. Danie, p 327 Residence times at lower tem- Brcalfhg Strength-grams fgi peratures, min 6 5. 25 3. 25 5 L "42 P10(]l:l)1Ct properties: 1 Modulus of Elaslicityg./dv l64 enier 1,426 ,381 1 377 c c mem 94.8 Breaking strength, grams 2, 070 2,469 1, 740 gimsiletstrength, g./d.* g5 Z9 26 A l W onga ion percent 6 6 8 Modulus oi'elasticlty, g./d 151 169 135 F. Phosphoric trlamlde, OP(Nl-l (applled as 3.5 per- Carbon content, percent" 92. 5 93. 1 90. 0 cent Solution) G ./d. =grams/denier.

Carbonization Temperatures 260 C.l .372 C. B. N,N,N"-Trimethylphosphoric triamide, OP(-NH ges'dence temperature 5.25 mm. CH (appl1ed as 3.5 percent solutlon) Carbonization temperature 'g f; Pmpemes l 306 CHI 1' 260 C.1372 C. 266 C.1372 C. Breaking Strength-grams 2.l86 Residence times at lower tempera- Tensile Strength-gjd. 1.66 mics, min 5 8- 4 E|nsarin L46 Modulus of Elasticit ./d. I68 Product propertlesz Y 3 Denier 1 110 1 189 Carbon Content-Z: 93.6 Breaking strength, g./d 1: 887 1,370 'Ensile strength, g./d 1. 70 1.15 I l Ongelion. D 26 7 G. Trimeth hos hate OP OCH a ied as a 5 er- Modulus of elasticity, g./d 156 165 p P pp p Carbon content, percent 97.0 95. 6 cent 501mm") Cnrbonization temperature C. (Chloromethyl)phosphonothionic diamide, 375a EEFQ 0.43725 W Residence times at lower tempera- SHNHZ)2 tures, min 7 4. 2 (applied as a 3.5% solution). 40 Prod i c t g p 010 890 A grealr liiE's'irii' gili'i idii'i'sI 1,834 1. 3% e ensi e strengt g./ 2. 2 gdrliionlzatlpn Temperatures 260 C.-l,372 C. Elongation perc'enL l 24 u. 86 em Modulus oielastielty, g./ 105 201 ""P Carbon content, percent U5. 8 J3. U

Product Properties iw 7 Denier L220 1,201 Breaking Strength-grams 2,250 2,358 OP(-OC2H5)3 Tensile Strength-97d. 1.42 L96 as a l ti Elongation-ii: l.l0 1.57 Modulus of Elasticity-gJd. I60 16s Carbonizaiion temperature 260 o.-1,372 0. 200 c.-1,372 6.

Residence times at lower tempcratures, min 5. 25 4. 0 5. 25

Carbonization Temperatures 266 C.l .372" C. qigfigiPffPffi 826 908 832 Residence Breaking strength, 1, 942 1, 640 1, 630 temperature 8.4 min. 10.5 min. Tensile strength, g./d 2. 35 1. 80 l. 96 Elongation, percent 1. 45 1. 00 1. 14 Modulus of elasticity, g./d 192 196 205 Product Properties Carbon content, percent 95. 2 U4. 6 06. 5

Denier L200 1,169 2-460 L837 1. Diethyl methylphosphoramidate,

Tensile Strengthg./d. 2.05 1.57 Elongation-i6 1.62 1.26 O P C2H5)2 refineries- 1: :2. (applied as a 5% solution).

Carbonization temperature D. Diammonium h dro en hos hate NH HPO a y p p 4 p 254 c.-1,372 0. 260 c.-1,a72 c. piled as 5 percent solutlon) Risidence times at lower temperaures, min 4.2 Carbonizalion Temperatures 260 C.-l ,372 C. I Residence times at lower Product properties: temperature 1 L6 min, Denier l, 072 1, 032 Breaking strength, grams. 2,118 232 iglcnsiletstrength, g./d 1. 2. P d P r onga lon, percent"... 1. 1.

f; Modulus ofclasticity, g./d. 183 175 Carbon content, percent 94.9 91. 7

J. Diethyl ethylphosphoramidate,

P(0C2H5)2 HN C2315 (applied as a solution).

carbonization temperature Residence times at lower temperatures, min 7 5. 25 3. 5

Product properties:

Denier 900 816 1, 012 Breaking strength, grams 2, 490 2, 452 2, 178 Tensile strengt g. 2. 77 3. 00 2. Elongation, percent- 1. 69 1. 83 1. 61 Modulus of elasticity, g 211 202 169 Carbon content, percent 99. 6 98. 7 95. 2

K. Diethyl isopropylphosphoramidate,

HN CH(CH3)2 (applied as a 5% solution).

Carbonization temperature, C 2601, 372 Residence time at lower temperature, min 5. Product properties:

Denier 830 Breaking strength, grams l, 302 Tensile strength, g./d 1. 59 Elongation, percent 0. 77 Modulus of elasticity, g./d 245 Carbon content, percent 93. 0

L. Methylphosphonic diamide,

01:01am CH3 (applied as 3%% solution)..

carbonization temperatures 271 C.1,372 C. 277 C.1,372 C.

Residence times at lower tempera- The results using fabric substrates are as follows:

M. A woven fabric consisting of 1650/720/22 rayon yarns in a 21x22 (warpXfilling) construction was impregnated with a 3.5 percent by weight aqueous solution of N,N', N' trimethylphosphoric triamide. After drying, the fabric was exposed to temperatures in the range of 288 C.-3 l6 v C. for a period of nine minutes. At this stage of carbonization the fabric had a carbon content of 60.8 percent. Other combinations of time and temperature produced the same result. The fabric was then passed through a zone heated to l,200 C. for a period of approximately 20 seconds. During this time the carbon level increased to 88 percent. The carbonized fabric was flexible and exhibited a high tensile strength in both warp and filling directions.

N. The same fabric wasTmpregnated with a 5 percent by weight solution of phosphoric triamide and exposed to a heating cycle consisting of a gradual increasing temperature from 230 C. over a period of fifteen minutes. The fabric was then exposed to a temperature of l,538 C. for a period of 20 seconds. The resultant carbonized fabric had a carbon content of 95 percent. The fabric had excellent tensile strength and was flexible and uniformly lustrous.

O. Comparable results were obtained on the same fabric treated with 5 percent by weight aqueous solutions of the following catalysts: phosphorothionic triamide, (chloromethyl)phosphonothionic diamide,

phosphonitrile amide cyclic trimer, trimethyl phosphate, triethyl phosphate, diethyl methylphosphoramidate, diethyl ethylphosphoramidate, diethyl isopropylphosphoramidate, and methylphosphonic diamide.

Since the disclosure in US. Pat. No. 3,294,489 (Millington) states that flameproofing agents generally, particularly phosphates, borates, and chlorides would be effective catalysts, several known flameproofing agents were evaluated and were not foundto be effective catalysts. For example, a boric acid-metaborate mixture, even at the 5 percent concentration level did not give a satisfactory product at carbonization temperatures of 260 C., 275 C., 288 C., at 10 minutes residence time followed by the usual 20 seconds at 1,372 C. Similarly, a borax-boric acid mixture at 5 and 10 percent concentration at 288 C. for periods of 6 and 9 minutes followed by exposure at the usual 1,372" C. for 20 seconds did not give a product of over 82 percent carbon and the product was too brittle for testing. The above data confirmed that the choice of catalyst even from the class of flame retardants is an empirical selection and successful catalysts cannot be predicted in advance.

As the above data indicate, the novel catalysts of this invention are advantageous in several respects. In some instances they offer advantages over prior art catalysts exemplified by diammonium hydrogen phosphate. For example, all of the catalysts can be applied at room temperature and most of the catalysts function well even at much shorter residence times in the initial carbonization. The lower temperature application is more convenient and reduces safety hazards while the lower residence times effect substantial savings in labor and product costs. The preferred compositions such as phosphoric triamide, phosphorothioic triamide, phosphonitrile amide cyclic trimer, diethyl ethylphosphoramidate, and diethyl methylphosphoramidates not only reduce residence time considerably but actually yield a product having superior physical properties.

As the various examples and discussion in the specification have previously indicated, numerous changes and modification of reagents and reaction conditions can be made without departing from the inventive concept. The metes and bounds ofthe invention are best described by the claims which follow What is claimed is:

1. A process for improving the thermal carbonization of cellulosic fibrous substrates comprising treating said substrates prior to carbonization with at least a catalytic amount of a catalyst of the formula R and R are selected from the group consisting of hydrogen and alkyl of l to 4 carbon atoms and heating said treated substrates until a carbonized product having a carbon content of at least 50 percent by weight carbon is produced.

2. The process of claim 1 wherein the treated substrate is heated until a carbonized product having a carbon content of at least percent by weight carbon is produced.

3. The process of claim 1 wherein the catalyst is phosphonitrile amide cyclic trimer.

* a: t =k UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 617,220 Dated November 2, 1971 I fl Giuliana C. Tesoro. Donald R. Moore and Stanley E. Ross It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

the Abstract, next to last line after "50 percent" insert by Col. 1, line 50, "200C. to 30 C." should read 200C. to 300%."; Col. 6, line 4, "(1969)" should read (1960) Col. 6, line 5, "cacording" should read according Col. 6, lines 13 and 15, "hezaamino" should read hexaamino line 15 at the end of the line "heza" should read hexa Col. 6, line 31 "75=25" should read 75:25 7 Col. 9, line 26, "1302" should read 1320 Col. 9, line 53, "316 vc. should read 316 C. Col. 10, line 1, "phosphorothionic" should read P phOrOthioic Signed and sealed this 19th day of March 1974.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents RM PO-1050(10-69) USCQMM-DC eoa-rs-peg v u 5 GOVERNMENT rnm'rmc OFFICE I965 0-366-334 

2. The process of claim 1 wherein the treated substrate is heated until a carbonized product having a carbon content of at least 80 percent by weight carbon is produced.
 3. The process of claim 1 wherein the catalyst is phosphonitrile amide cyclic trimer. 